Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Tue, 14 Jan 2003 10:16:46 -0500
Message-ID: <redirect-1983010@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: wing incidence and wing loading
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I know most are interested in the IV wing, but my experience was with the
ES.  To answer the questions from Mike one at a time see below:

<<Hi Guys,

<<I am a bit surprised to learn that people are "finding" that their wing
incidence is different from side to side by as much as 2 degrees or that the
wing twist (washout) varies by more than 0.5 degrees. These are pretty
significant differences for such critical dimensions. I have several
questions for builders that have encountered these variances:

<<1. Is this problem unique to the L-IV, or has this problem cropped up with
the other models as well?

Ans:  My ES had pretty much the problem noted above.

<<2. Are these problems particular to fastbuild wings or slowbuild wings?

Ans:  Mine was with the "fastbuild wings"

<<3. Are any of these problems occurring on aircraft built at the Lancair
builders workshop?

Ans:  Yes.  The first time I saw my wings on the aircraft was after I got
them home and that's when I "discovered" the problem.  They were closed at
the factory in their jigs.  One thing I noticed (January 01) was that the IV
wing jigs were steel while the ES jigs were wood - mine was the first or
second "builder's assist" project to use the jigs.  Since then I have looked
at other ES wings and most seem to have the same characteristic - one wing
(forgot which) has a slight droop, or "gull wing" effect, that starts at
about the aileron/flap junction.  I assume this is due to some defect in the
mold or jig.

<<4. Most importantly, what went wrong and how can it be prevented?

Ans:  I have no answer for this one.

<<I have heard from another builder that the fastbuild wings can vary
significantly in the measured washout vs. what is called out in the plans.
Has anyone else found this to be true? Are these issues a good reason to
consider passing on the fastbuild wings and rigging your own jigs for the
slowbuild wings?

Ans:  I think I would rather wrestle with the problem than spend the time to
build my own and then maybe have other problems to solve.

<<As I get closer to putting down my deposit for an L-IV, these are some
pretty
fundamental issues that should be considered by any prospective builder.

Thanks in advance for your replies.

Best Regards,
Mike Hutchins>>

Another post:

<<Other than being
heavier and being in different places within the CG range, what other
(non-emergency) flight characteristics develop between 3,000 and
4,000#'s? Your point about emergency situations is well-taken, and I
wish I knew where the "safe" emergency situation GW/CG was.

Brian>>

Larry Graves had a nice explanation.  To add to it, I think there is another
significant problem with higher gross weights, but it occurs at the forward
CG limit.  The tail has to be able to provide sufficient downforce to be
able to raise the nose to a proper landing attitude, and that requirement is
often the limiting factor for forward CG location.  Since, especially at the
forward CG condition, this downforce increases with increasing weight there
will be a weight at which a nose-high landing cannot be made.  Presumably,
the tail will only run out of "lift" under these conditions and not stall.
But during normal stall testing how does one know that the wing stalled and
not the tail?  I don't know.  If the tail stalled at the forward CG limit
you could end up suddenly dropping it in on the nose gear.  Another problem
is rotation at takeoff, which requires the nose to be "levered" up using the
main gear as the pivot point.  At a higher-than-designed weight the takeoff
speed may be determined by the speed at which the tail can develop enough
downforce to lift the nose.  Take a look at a typical Cessna or other
certified aircraft CG envelope and you will usually see the forward CG limit
to have a break in it so that the forward limit is further back at high
weights - it is for just these reasons.

Just for example, look at the takeoff performance as the weight is
increased.  First, the acceleration is less because the thrust is the same.
A 10% higher weight will result in a 10% lower initial rate of acceleration
(neglecting rolling resistance increases).  Besides that, the aircraft must
be accelerated to a higher speed, so the takeoff time will be more than 10%
longer.  You might think that since the takeoff speed is 5% higher (square
root of the weight) the takeoff time will be 15% longer.  Not so, as only
the horsepower is constant, not the thrust.  Thrust will go down somewhat
because of the higher speed, depending on the propeller efficiency
characteristic.  This might add another 5% to the takeoff time.  Now if the
speed required to lift off is further increased by the lift capability of
the tail the time could be increased even further.  Further, we are usually
looking at takeoff distance, not time, as the constraint.  Takeoff distance
will increase very roughly as the square of the takeoff time and the time
went up by maybe 20%, making the takeoff distance increase by something
probably 40% more or less.  Of course, with lots of horsepower available
that might still be an acceptable number.  For those of us that are
horsepower limited there is the effect of greater induced drag caused by the
higher weight.  The aircraft may be able to accelerate to what would
theoretically be flying speed, but then not have the power to overcome the
induced drag necessary to carry the aircraft, especially out of ground
effect.  I could demonstrate that effect in my Cardinal, but I don't think
any Lancair model will be much limited by that problem.  And you turbine
guys, when asked about your wingspan immediately think "80 inches" as you
don't need the wing anyway, just the prop....

Gary Casey
ES project, progressing ever so slowly